1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus that employs the sheet conveying device.
2. Description of the Related Art
In an image forming apparatus, an original reading apparatus, and the like, various kinds of sheet-like members such as a printing paper, a thin paper, a thick paper, a postcard, and an envelop (hereinafter collectively, “sheet”) are used. With such a wide variation of the sheet, when a sheet thicker than a certain thickness enters into a pair of conveyor rollers, a fixing unit, or a transfer unit, the operating speed of a sheet conveying unit that has been operating at a constant rate drops temporarily, leading to various problems such as image distortion. A conveying roller, a fixing roller, an image carrier, and an intermediate transfer unit are exemplified as the sheet conveying unit.
Specifically, an intermediate-transfer-type image forming apparatus may have image distortion at a primary-transfer unit due to temporal speed drop of the intermediate transfer unit when a sheet thicker than a certain thickness enters into the conveying rollers or a secondary-transfer unit.
Furthermore, for a color image forming apparatus configured such that the secondary-transfer unit and the fixing unit are closely arranged to each other to downsize the apparatus, transferring and fixing of an image is being concurrently performed on a sheet. In other words, when an image is being transferred onto a trailing edge of a sheet, an image that has been transferred onto a leading edge of the sheet is fixed. In such a color image forming apparatus, distortion may occur in the image at the secondary-transfer unit due to temporal speed drop of a fixing roller or a fixing belt when a sheet thicker than a certain thickness enters into the fixing unit.
Moreover, in a concurrent transferring/fixing image forming apparatus that performs transferring and fixing of a toner image onto a sheet at a time, distortion occurs in the image at the primary-transfer unit and the secondary-transfer unit due to temporal speed drop of the intermediate transfer unit when a sheet thicker than a certain thickness enters into the transferring/fixing unit.
To address these problems, Japanese Patent Application Laid-open No. 2006-85153 discloses a technology in which speed of an endless belt is kept constant by varying amount of speed control of a driving source for the belt based on a predetermined timing, a predetermined amount, and a predetermined time.
However the technology in Japanese Patent Application Laid-open No. 2006-85153 has a problem in that optimum control on all usable sheets is difficult because preset control target values are used corresponding to the thickness or the kind of the sheets.
Even the same sheet may have a change in its texture or in thickness depending on an environment to be used such as humidity, causing different types of the speed fluctuations, so that optimum controlling of the speed fluctuation is difficult. Furthermore, a storage area for storing such control target values to deal with various kinds of sheets is needed. For improving capabilities of handling sheets, a storage unit having a larger storage capacity is required.